In a voice/data switching system based on the International Telecommunications Union (ITU-T) V.8bis standard, a pair of tones are used to switch an ongoing voice connection from a voice mode to a data mode. The voice connection may be established between a pair of users over a telephone line, ISDN line or other suitable communication medium. Each of the users has a modem which is configured to monitor the ongoing voice connection. When one of the users needs to send data over the voice connection, its modem first transmits a predetermined pair of tones over the connection. The modem at the other end of the connection is continually monitoring the connection for the presence of the pair of tones, and upon detecting the tones takes appropriate actions to enable it to receive and process the data.
Two related problems which can arise in detecting a pair of tones in a V.8bis system are false detection or "talkoff," in which a voice or background noise signal is falsely detected as the tone pair, and missed detection or "talkdown," where the presence of voice prevents detection of a received tone pair. These problems can be partially alleviated by appropriate selection of tone frequencies. For example, the selected tone frequencies should be different than the frequencies of most speech signals. However, design of a dual tone detector for the tone pair typically involves a tradeoff between the talkoff and talkdown failure modes. Although many other systems, such as dual-tone multiple-frequency (DTMF) systems, also rely on dual tone signaling to perform various functions over a voice connection, systems based on the V.8bis standard are particularly problematic in that the tones must be detected in the presence of either voice or silence, where the silence is actually low-level background noise.
A conventional dual tone detector includes a pair of narrow filters, with each of the filters tuned to one of the tone frequencies. The tone pair is determined to be present if the output energy of both filters exceeds a threshold for a predetermined period of time. However, this conventional approach is unable to adequately resolve the above-noted talkoff and talkdown problems, particularly if the received signal strength is highly variable as is typical of dial-up voice connections. If the threshold and time period are set too low, or the received signal strength is too high, false detection will be likely. Conversely, if the threshold and time period are set too high, or the received signal strength is too low, legitimate tone pairs may be missed. One known enhancement of the basic conventional dual tone detector involves disabling the detector when the total received signal strength is below a threshold. Other enhancements are based on taking Fourier transforms of the received signals. Unfortunately, these and other known enhancements remain unable to provide adequate tone detection in V.8bis systems and other important applications. A need therefore exists for an improved multiple tone detector which can better prevent false detection and missed detection than the above-described prior art techniques.